The following note has been written partly by way of a commentary upon Dr. H. C. Watt's recent article, in the July number of the Aëronautical Journal, entitled, “ A Note on the Theories of Screw Propulsion,” and partly by way of an explanation of the apparent inconsistency of the “inflow” theory of the airscrew.

Consider an element of a propeller blade situated at a radius of x from the boss centre of the propeller and having an angle of ϕ between the chord line of the blade section and the plane of rotation. The form of the blade section is considered to be that of an aerofoil, of which the characteristics are known from wind channel experiments upon a model of the same geometrical form (Fig. I).

We wish to apply these wind channel data to the propeller blade element with rigorous consistency, i.e., as far as the two respective régimes of the propeller and the aerofoil permit.